An LED is a rectifying semiconductor device which converts direct current electrical energy into light energy. Unlike most incandescent or fluorescent lights, an LED operates using only a direct current (DC) or a one-way flow of electrical energy as opposed to a two-way flow of alternating current (AC) of electrical energy where the polarity and the direction of the flow of electrical energy reverses many times per second.
Each LED has a positive polarity connection or anode and a negative polarity connection or cathode. The direct current or the forward flow of electrical current flow is from the anode of the LED to the cathode of the LED.
Typically, in prior art strings of series-connected LEDs 100, individual LEDs 22 are normally assembled, one to another, in a series circuit 20 as shown in FIG. 1. Therein, it is shown that the anode 24 of the first LED in a string of series-connected LEDs is connected to the positive polarity 102 of a direct current power supply. The cathode 26 of each LED is connected to the anode 24 of the next LED in the string of series-connected LEDs. The cathode of the last LED in the string of series-connected LEDs is connected to the negative polarity 104 of the direct current power supply, thereby closing the direct current circuit and causing each LED in the direct current circuit to emit light energy.
As in any series-connection of electrical components, a break in the series connected string of LEDs renders the circuit no longer conductive, thereby no electrical energy will flow from the positive polarity of the power supply to the negative polarity of the power supply.
When a single LED in a string of series-connected LEDs fails, none of the LEDs in the string of LEDs will emit light energy as there can be no flow of electrical energy from the positive polarity of the direct current electrical energy power supply to the negative polarity of the direct current electrical energy power supply.
If a string of series-connected LEDs is used as a light source, for example in an illuminated sign, the light source which illuminates the sign is typically multiple strings of series-connected LEDs. Each of the strings of series-connected LEDs contains multiple individual LEDs. If one individual LED in a string of series-connected LEDs fails, this is called an LED open circuit. The string of series-connected individual LEDs containing the failed LED does not provide a closed circuit from the positive polarity of the direct current electrical energy power supply to the negative polarity of the direct current electrical energy power supply. Accordingly, none of the LEDs in the string of series-connected LEDs will emit light energy. The loss of an entire string of series-connected LEDs will significantly reduce the total light output of a lighting system; including multiple strings of series-connected LEDs, and create a noticeable dark spot on the surface of an illuminated sign. When there is a loss of a complete string of series-connected LEDs in a device such as an illuminated sign, the manufacturer may be required to disassemble the sign and replace one or more complete strings of series-connected connected LEDs. Such need to replace a complete string of series-connected LEDs increases manufacturer's warranty costs and user maintenance costs.
As shown in FIG. 2, the problem of a single failed LED in a string of series-connected LEDs is typically solved in the prior art by placing a bypass circuit 30 in parallel with or around an individual LED in a string of series-connected LEDs. Thus, when a single LED fails in a string of series-connected LEDs, the direct current flow of electrical energy passes through the bypass circuit around the failed LED and on to the operating LEDs positioned after the failed LED in the string of series-connected LEDs. For a multi-LED light source, the loss of a single LED in a string of series-connected LEDs makes only a small difference in the amount of illumination provided by multiple series strings of LEDs. Thus, if there is the loss of but a single LED, the manufacturer of the illuminated sign will typically not be required to replace an entire string of series-connected LEDs.
The prior art bypass circuits shown in FIG. 2 are configured to be used with each individual LED in a string of series-connected LEDs. In normal operation, an individual LED is conductive; that is, the individual LED allows electrical current to pass therethrough. But when an LED open circuit condition is created, no electrical energy flows through the individual LED. Instead the electrical energy flows through the conductive bypass circuit formed around the failed LED. That is, in normal operation the electrical componentry in the bypass circuit around the LED renders the bypass circuit non-conductive. It is only when an individual LED fails and will not allow any electrical energy to pass therethrough that the electrical energy which formerly passed through the LED activates electrical componentry in the bypass circuit and makes it conductive to the flow of electrical energy. Once activated, the bypass circuit enables the passage of electrical energy around the failed LED to the next individual LED in the string of series-connected LEDs so that the next individual LED in the string of series-connected LEDs operates normally and the flow of electrical energy through the string of series-connected LEDs is restored.
Prior art bypass circuits for use in a series string of LEDs typically include a zener diode. A zener diode is a two-terminal semiconductor junction device which is normally non-conductive to the flow of electrical energy; that is, no electrical energy passes therethrough. However, when electrical current of a predetermined voltage is applied to the zener diode, the zener diode becomes conductive; that is, the zener diode allows electrical energy to flow therethrough.
When an LED is operating normally, that is emitting light energy, the forward voltage needed to cause the zener diode (about 4.2 volts) to become conductive to the flow of electrical energy is higher than the forward voltage needed to enable the flow of electrical energy through an operable individual LED (about 3.2 volts). Thus, electrical energy flows through the individual LED and does not flow through the zener diode when the individual LED is operating normally. When an LED fails, the electrical resistance of the failed LED goes to infinity, and no amount of electrical energy or forward voltage will enable electrical energy to pass through the failed LED. Accordingly, the voltage from the direct current electrical energy power supply will flow to the zener diode. The electrical energy voltage from the direct current electrical energy power supply will cause the zener diode to become conductive to the flow of electrical energy therethrough. When the zener diode in a bypass circuit is conductive, the bypass circuit is active so that a current path around the failed LED or bypassing the failed LED is provided. As explained above, by the use of prior art bypass circuits, including a zener diode, the loss of a single LED will not shut down an entire string of LEDs.
While the use of a zener diode will enable the electrical energy from a direct current electrical energy power to bypass a failed LED in a string of series-connected LEDs, the use of a zener diode also presents certain problems which can affect the operation of a series-connected string of LEDs. These problems come from the heat energy generated by a zener diode. This heat energy is higher than the heat energy generated by an LED. When the amount of electrical current needed by the string of series-connected LEDs is low, such as when there is a small number of LEDs or when the light output from the LEDs is low, the generation of heat energy is not usually a problem. But when the amount of current needed by the string of series-connected LEDs is high, the generation of heat energy caused by the use of a zener diode becomes a problem, and a heat sink may now be needed to dissipate the heat energy emitted by the zener diode. The use of a heat sink to dissipate the heat generated by multiple zener diodes in the bypass circuit around each individual LED increases the size, the weight and the cost of an LED light source system.
In addition, when the bypass circuit around each individual LED is activated, the total voltage across the string of series-connected LEDs increases. This increased voltage increases the electrical energy consumption of the string of series-connected LEDs. If enough bypass circuits in a string of series-connected LEDs are activated, the total electrical energy available from the direct current power supply may be exceeded or the maximum allowable direct current voltage from a constant amperage power supply may be surpassed. In such cases, this need for additional electrical energy or additional voltage may shorten the life of the constant amperage direct current electrical energy power supply.
Another problem occurs when a string of series-connected LEDs is improperly connected with respect to the polarity of a direct current power supply. Specifically, a portion of the string of series-connected LEDs which should be connected to a positive side of the direct current power supply is connected to the negative side of the direct current power supply. When this occurs, the LEDs will fail and the entire string of series-connected LEDs which is improperly connected to a direct current power supply will have to be replaced. To prevent the failure of the string of series-connected LEDs, there is a need to protect each LED in the string of series-connected LEDs with a bypass circuit.
Accordingly, there remains a need in the art for a low power bypass circuit connected in parallel around an individual LED which becomes operative when an LED fails or becomes operative when a string of series-connected LEDs has been improperly connected to a direct current power supply. Further, such low power bypass circuit should enable the use of minimal electrical energy so that a heat sink for dissipation for generated heat energy is not required nor is a voltage which exceeds the normal operating capacity of the direct current power supply required.